Preheat the oven to 350°F (175°C).
In a mixing bowl, combine 4 cups of chopped rhubarb with 1 cup of granulated sugar and 2 tablespoons of all-purpose flour.
Mix well and set aside to allow the flavors to meld.
Combine the topping ingredients in another bowl.
Mix together 1 cup of old-fashioned oats, 1 cup of all-purpose flour, 1/2 cup of packed brown sugar, and 1/2 cup of melted butter along with 1/2 teaspoon of cinnamon if desired.
Spread the rhubarb mixture evenly in a baking dish.
Top it generously with the oat mixture, distributing it evenly over the rhubarb.
Bake in your preheated oven for about 30 minutes.
Look for a golden-brown topping and bubbling rhubarb as an indicator it's ready to come out.
Cool slightly before serving.